XRAY - Model racing cars

My motor is quite hot when it comes off after 5mins
I am running a hobby wing 17.5t motor with a fan. I haven't had a temp gun on it so can't give a exact reading but is hot to the touch. I am geared at 3.8 which is around what the others are geared at but with different motors.

So the question is would I go up or down a toothe on the gearing to get in down a bit on the heat but not lose the performance

You'd gear down (smaller pinion) or reduce the timing to reduce the heat. Reducing the timing is usually the best way of cutting heat for the smallest loss of laptime.

But you should get a temp gun. Any motor is going to feel hot after a race. If you can touch it without burning your finger it's probably okay! 70deg is a safe limit for most motors. Some can manage a lot more than that.

The fact that you cannot touch it does not necessarily mean that it is too hot. Some motors like LRP/Nosram work fine up to 100 degrees. I would advise you to use a temp gun.